This invention relates to redundant inertial reference units employing a symmetrical gyroscope configuration.
It has previously been proposed to use redundant systems of gyroscopes arranged along non-orthogonal axes in such a manner that the system could suffer a failure of any two gyroscopes and still be capable of determining the essential location of a vehicle in inertial space. In this connection, and as a general background reference, attention is directed to "A Redundant Strapdown Inertial Reference Unit (SIRU)", by Jerold P. Gilman and Richard A. McKern, Journal of Spacecraft, Vol. 9, No. 1, January 1972, pp. 39 to 47. This reference discloses a system in which six single-degree-of-freedom gyroscopes are employed in an arrangement where the spin vectors of the gyroscopes form an array of perpendiculars to the faces of a dodecahedron, a twelve sided solid figure having six pairs of opposed parallel faces. Such a symmetrical non-orthogonal arrangement of spin axes in an inertial reference unit allows up to two of any of the gyroscopes to fail and still have sufficient inputs available from the remaining gyroscopes to isolate the failures, disregard their output signals, and determine the location of the system in a coordinate system in inertial space. The symmetry of the system allows modular installation of each gyroscope and accompanying apparatus in the reference unit so that each module is interchangeable with all others and the system may function with up to two modules removed.
Another proposed arrangement using four two-degree-of-freedom gyroscopes is described in NASA report No. NASA CR-132419, "Investigation of Application of Two-Degree-of-Freedom Dry Tuned-Gimbal Gyroscopes to Strapdown Navigation System," by Teledyne Systems Company, Northridge, Calif. In this report, an arrangement is described in which four two-degree-of-freedom gyroscopes are utilized. Three of the gyroscopes are arranged with their spin axes orthogonal to each other. The fourth gyroscope's spin axis forms substantially the same angle with each of the other three axes. This configuration is also capable of supplying sufficient data to isolate and disregard erroneous output signals from any two of the gyroscopes and still have sufficient data from which to determine location in inertial space.
The principal advantage of the four two-degree-of-freedom gyroscope configuration over the above described dodecahedron arrangement is the reduced number of gyroscopes. However, the four gyroscope configuration sacrifices symmetry and thereby eliminates the possibility of the modular packaging approach of the dodecahedron configuration.